Controlled-source electromagnetic (“CSEM”) geophysical surveys use man-made electromagnetic (“EM”) fields to evaluate the presence of resistive strata within the earth. The resistance can be caused by salt, coal, freshwater or hydrocarbons. CSEM techniques currently do not provide conclusive evidence of hydrocarbons and are used in conjunction with other geophysical and geological data. CSEM surveys typically record the EM signal induced in the earth by a source (transmitter) and measured at one or more EM sensors (sometimes called detectors or receivers), deployed on the earth's surface, the seafloor or inside boreholes. The behavior of this signal as a function of transmitter location, frequency, and separation (offset) between transmitter and receiver can be diagnostic of rock properties associated with the presence or absence of hydrocarbons. Specifically, CSEM measurements are used to determine the spatially-varying electrical resistivity of the subsurface. This technology has been applied in tectonic studies, hydrocarbon and mineral exploration, environmental and geological engineering
In the marine environment, CSEM data are typically acquired by towing a Horizontal Electric Dipole (HED) transmitting antenna 11 among a number of autonomous sea-floor receivers 12 positioned on the seafloor 13. (See FIG. 1) The receivers typically have multiple sensors designed to record different components of the electric and/or magnetic fields. The transmitter antenna is typically towed a few tens of meters above the seafloor. The receivers are weighted and fall to the seafloor, but are buoyant enough to rise back to the surface with the data after receiving an acoustic command from the ship.
Alternative dipole (source) configurations include:
laying the HED on the sea-floor and transmitting a waveform for several minutes to a few hours,
suspending the Electric Dipole vertically in the water column (Vertical Electric Dipole (VED)) and transmitting a waveform for several minutes to a few hours,
towing a VED (see PCT Patent Publication No. WO 2005/081719 A2), and
using a magnetic dipole source.
The transmitting and receiving systems typically operate independently (without any connection), so that receiver data must be synchronized with shipboard measurements of transmitter position by comparing clock times on the receivers to time from a shipboard standard (typically GPS derived UTC (Coordinated Universal Time)).
The energy transmitted by the source is in the form of a continuous high power current. The amplitude and frequency of the current output is controlled to generate a variety of different waveforms, including:
sine waves;
square waves;
Cox waves (J. Geophys. Res. 101, 5519-5530 (1996));
Pseudo-random binary sequences (Geophysics 45, 1276-1296 (1980));
logarithmically-spaced multipeak waves (PCT Publication No. WO2005/117326); and
time division multiple waveforms (PCT Patent Application No. PCT/US06/33695).
A specific waveform is typically designed or selected for each survey area to provide an optimal combination of depth penetration and frequencies. The transmitted waveforms are typically generated using Alternating Current (AC) signals with high current and low voltage. In order to process CSEM survey data and interpret the data, it is necessary to know the EM signal being transmitted. Typically, instrumentation is used to monitor the transmitter for this purpose. The key components of conventional waveform monitoring are illustrated in FIG. 2, which is a schematic representation of a CSEM Horizontal Electric Dipole (HED) source. The main pressure vessel 21 will house the switching hardware 22 which generates the specified waveform from a high current (>500 A), low voltage (˜100V) AC or DC input signal. The two electrodes, a “near” electrode 23 and a “far” electrode 24, are attached to the switch outputs via a streamer or dipole 26. Typical distances from the pressure vessel are ˜20-30 meters and ˜100-300+ meters for the near and far electrodes, respectively. Conventional waveform monitoring is performed using a current clamp, Hall effect transducer or equivalent, which generates a calibrated output current based on an input electric or magnetic field. The possible sensor locations 25 for such a conventional monitoring device are illustrated; they capture the waveform at the pressure vessel and not the waveform physically transmitted by either electrode.
The actual transmitted signal must accurately represent the design waveform to meet the chosen criteria. Typical transmitted waveforms are shown in FIGS. 3A and 3B. FIG. 3A shows an 8-second square wave generated by a source called DASI-II (originally developed by Cambridge University, England, UK). The high current signal is generated as a 256 Hz sine wave, which is rectified and the resulting 512 Hz half sinusoids are switched via a bridge to generate the desired waveform. The insert 31 shows an exploded view of the first second of the square wave, which reveals that the 512 Hz half sinusoids are present in the transmitted waveform. All one-sided transmissions exhibit a reduction in transmitted current (˜5%) which is believed to be an electrical limitation of, or electrochemical reaction at, the electrodes. FIG. 3B shows an 8-second square wave generated by a source designated DASI-III (developed by O.H.M. Limited, Scotland, UK). The exploded view in the insert 32 shows a ripple at ˜360 Hz present in the transmitted waveform, as a result of rectifying 3-phase 60 Hz A/C power.
FIG. 4A shows on a much longer time scale a source signal 41 generated to by a CSEM source complete with control circuitry to ensure constant transmitted voltage. The depth of the source dipole's midpoint is shown by 42. The transmitted current is a function of transmitted voltage÷resistance of the medium surrounding the electrodes, and resistance is approximately inversely proportional to salinity, the major factor that influences resistance. The short term change in HED depth (at ˜304.875 Julian days) coincides with the source crossing a sea floor channel. As the source maintains a constant altitude above the sea-floor, the water depth increases (i.e. becomes deeper) at the channel, causing salinity to go down (salinity decreases with water depth), resistance to increase, and consequently transmitted current to decrease. FIG. 4B shows source signal over a long time frame for another constant-voltage CSEM source, and FIG. 4C shows the corresponding source depth. Discontinuities in the transmitted current such as 43 are due to operator reduction 44 of the input voltage (FIG. 4D).
The transmitted amplitude may experience both short term (FIGS. 3A and 3B) and long term (FIGS. 4A and 4B) variations, which must be captured and fully compensated for during data processing. The same is true when the CSEM source has control circuitry to ensure constant transmitted current, as shown in FIG. 4E. The points plotted in FIG. 4E represent five different source lines of different durations: 6, 14, 15, 16 and 20 hours. This demonstrates the repeatability of CSEM sources that incorporate additional control circuitry. The repeatability in this example is better than 0.1%.
An attempt to monitor the transmitted waveform by means other than the current clamp 25 of FIG. 2 is detailed by MacGregor in Electromagnetic investigation of the Reykjanes Ridge near 58 North, Ph.D. Dissertation, Cambridge, pages 63, 79 and 82 (1997). This attempt is described as follows: “A mini-streamer containing four electrodes placed at 22, 48, 88, and 89 m behind the “DASI” (Deep towed Active Source Instrument) is attached to the main array to monitor the transmitted fields. Voltages between the outer pair (22 m and 89 m) and inner pair (48 m and 88 m) of electrodes are recorded on two channels of a data logger mounted on DASI.” This solution was technology limited, stated as follows: “Data storage limitations in the piggyback logger meant that the source fields were logged for at most three minutes in every half hour of transmission.” The results of the waveform monitoring are described: “During the first tow, the piggyback data logger recorded the source fields for only three minutes in every hour. In order to reconstruct the transmission pattern, the times at which the source frequency changed are required. Insufficient frequency transitions were logged to be able to use the piggyback logger to reconstruct the transmission pattern.” Further: “To establish the transmission pattern during tow 2, frequencies and transition times were measured from the piggyback logger record. This provides an incomplete record of transmission since it logged for only three minutes in each half-hour. However, enough frequency transitions were detected for the record to be used in reconstructing the source pattern.” Not long thereafter, the current clamp, for example a Hall Effect transducer, located essentially as illustrated by 25 in FIG. 2, became the widely accepted way to monitor the transmitted waveform, and will be referred to herein as “conventional waveform monitoring.”
The present inventors discovered a problem with conventional waveform monitoring during examination of CSEM data from a survey conducted during the period Jul. 7, 2003 to Dec. 2, 2004, which led to the present invention as described herein below.